1. Field of the Invention
The present invention relates to a fluorine-containing aromatic compound, a process for preparing the same and use thereof.
2. Description of the Prior Art
Some aromatic compounds comprising two benzene rings and two trifluoromethyl groups are known. For example, U.S. Pat. No. 3,959,350 discloses a compound of the formula: ##STR5## J. Polym. Sci., B-3, 1021 (1965) discloses a compound of the formula: ##STR6##
These known compounds are useful as starting materials in the production of fluorine-containing epoxy resins and fluorine-containing polyimides.
However, a compound of the formula (1) or (2) in which at least one of the trifluoromethyl groups is replaced with a perfluoroalkyl group having at least two carbon atoms has not been known. This may be because replacement of trifluoromethyl group with the perfluoroalkyl group having at least two carbon atoms is very difficult due to steric hindrance.
Hitherto, as a fluorine-containing epoxy resin, a polymer of an epoxide of the formula: ##STR7## is known (cf. J. Polym. Sci., B-3, 1021 (1965)). The cured product of this polymer has a refractive index n.sub.D.sup.23 of 1.524 to 1.527. When an epoxy resin is used as an adhesive for a quartz optical fiber, it is advantageous that the adhesive has a refractive index as close as possible to that of the quartz. (n.sub.D.sup.23 =1.46). Therefore, the epoxy resin (3) is not satisfactory as an adhesive for optical parts or elements such as the quartz optical fiber and an optical connector.
Polyimide resins are known to have good heat resistance and electric insulation property, and used as flexible substrates for printed circuits, a protective coating for various elements, an adhesive for bonding a chip to a substrate and the like. However, the conventional polyimide is highly hygroscopic. Therefore, it causes corrosion in these applications and its adhesiveness decreases. To improve the hygroscopicity of polyimide, fluorine-containing polyimides comprising repeating units of the following formula are proposed: ##STR8##
Although these fluorine-containing polyimides have less hygroscopicity than the conventional polyimides, they still absorb about 0.5 to 1.0% of water. Therefore, a polyimide having much less hygroscopicity has been desired.
In order to improve moisture resistance of a semiconductor device and to prevent software errors caused by alpha-ray, it is known to provide a protective coating of a polyimide resin on a surface of the semiconductor device. Also, a semiconductor device having multilayer wiring in which the polyimide layer is provided between the adjacent layers is known. However, as described above, since the conventional polyimide has large hygroscopicity, the device tends to be corroded or adhesiveness is deteriorated. Practically, the corrosion or the deterioration of adhesiveness results in corrosion breaking of an aluminum or copper wire used for wiring of LSI, blister of an insulating layer by abrupt heating during soldering or in a bonding step, and leakage current in case where the polyimide is used as a surface stabilizing layer on an exposed end of PN junction. Then, it has been proposed to use a fluorine-containing polyimide having improved hygroscopicity as a protective film for the semiconductor device. For example, Japanese Patent Kokai Publication No. 177659/1985 discloses a method for producing a semiconductor device comprising applying a polyamic acid which is prepared by reacting dianhydride of a tetracarboxylic acid of the formula: ##STR9## and a diamine of the formula: ##STR10## and then curing the polyamic acid.
By the above method, the hygroscopicity is not satisfactory and the problems caused by hygroscopicity are not solved.